Parasitic resonances in the controlled system cause problems in the control of machine tools, robots or other production machines, hereinafter machines. These problems are usually rendered harmless by the use of filters. However, even compensation with the aid of filters has problems. The phase consumption of the filters reduces the achievable controller gains, which corresponds to a dynamic loss for the controlled system. This is undesirable particularly in the present-day highly dynamic electric drives, because a portion of the design-induced dynamics available through the drives is lost.
Filters are typically used in rotational speed controls. The illustration according of FIG. 1 shows a block diagram of a control structure for controlling an electric drive. What is shown is a drive control A with a position controller L, a downstream rotational speed controller D and a downstream current controller S with corresponding desired values 1* for the position, n* for the rotational speed and i* for the current. A motor M is controlled thereby via a power supply system LT, an inverter with a rectifier, intermediate voltage circuit and a transducer with power transistors. For positioning purposes, the rotational speed controller D is subordinated to the position controller L for example for a feed drive or a C-axis of the main spindle motor. Corresponding actual current values ia and ib are fed back from the power supply system LT to the current controller. The third phase of the three-phase current motor M is calculated. Fitted on the axis of the motor M is a transmitter system with a tachometer G and a position encoder LG, which return an actual rotational speed value n to the rotational speed controller D, and an actual position value 1 to the position controller L.
The illustration in FIG. 2 shows a control loop model of the block diagram system shown in FIG. 1, having a preprocessing unit L to which the desired rotational speed values n* are applied, a rotational speed controller D, and a current controller S, which receives desired current values i* from the rotational speed controller and supplies actual current values i. The latter are fed via a KT element for generating instantaneous values m to the motor M, which generate actual rotational speed values n which are fed back negatively to the input of the rotational speed controller D. The same holds for the actual current values i, which are likewise fed back to the input of the current controller S. Further details of the rotational speed controller D, are shown in FIG. 3. The controller D comprises a PI controller with proportional and integral components, downstream of which the filter F is connected.
In current systems, either individual resonances within such a control loop have been specifically damped by filters F in the form of band-stop filters, or the filter action has been set over a large frequency range by means of a lowpass filter (PT1 or PT2 element) customary in control engineering. FIG. 4 shows the amplitude profile A(f) and the phase profile φ(f) of a band-stop filter, plotted against frequency f. PT1 and PT2 elements cause a reduction in amplitude which increases with rising frequency f and is active even at high frequencies where, because of the lowpass response of electric drives, a reduction would not be necessary. This unnecessary reduction is attended by additional phase losses at low frequencies. This relationship can also be seen from the amplitude profile A(f) and phase profile φ(f), shown in FIG. 5, of a PT2 element.
Damping of individual resonances by means of band-stop filters therefore causes a relatively low phase loss, and thus dynamic losses are not excessively large. However, the robustness of such a solution is poor. This is problem in systems with varying resonances, which can occur, for example, from system aging, alteration of tools or workpieces, altered machine geometry or during the processing stage (tool engagement). Thus the controlled system can become unstable or is poorly damped.
In the direct drives being increasingly used in automation engineering, resonant frequencies frequently vary particularly strongly during the traversing operation, and therefore cannot be sufficiently reduced by band-stop filters. Band-stop filters are therefore not a robust solution to stop these unwanted resonances, since only a narrow frequency range is filtered. In the case of linear drives, the controlled system is mostly characterized by many resonant frequencies which are situated very closely next to one another and can in practice be effectively suppressed only with the aid of lowpass filters.
Conventional lowpass filters admittedly are a robust solution, ensuring stable behavior even in the case of varying resonances, but they cause a substantial loss in dynamics which cannot be tolerated especially in the case of direct drives. Even slight phase drops markedly slow the operation of the controlled system. A relatively large phase loss, such as a loss of 30 degrees, transforms the dynamic, fast drive into a slow system. Because of the severe phase drop, it is therefore necessary to accept a low controller gain and, as a result thereof, an inferior suppression of interference and a slower response to setpoint changes.